1. Field of the Invention
The present invention relates to the general area of generating carriers such as electrons and holes within semiconductors by the action of incident radiation, being more particularly concerned with radiation emanating from heated surfaces, and, in an important application, to the enhancement of such generation within photovoltaic devices and the like, due to the close proximity of the heated surface.
2. Description of the Prior Art
In a common photovoltaic cell, a semiconductor p-n junction is formed close to the surface of the semiconductor material that forms the cell. When photons emitted by a light source such as the sun impinge on the cell surface, electron-hole pairs are created. These electron-hole pairs are separated by the space-charge potential that is a consequence of the p-n junction. The net result is a DC current. Thermophotovoltaics operate in a similar manner except that, instead of a light source, a surface at a higher temperature than the semiconductor material acts as the source of photons. In this case, thermal radiation is the mechanism of energy transfer and the temperature of the emitting surface which dictates the spectral composition of the radiation must be matched to the material and electronic properties of the semiconductor such as its bandgap in order to optimize conversion efficiency.
Prior thermophotovoltaic devices and systems have been designed such that the distance between the emitting surface and the cell surface is large relative to the characteristic wavelength of the thermal radiation. Hence, the thermal radiation transfer is characterized by the Stefan-Boltzman Law and its spectral composition by Planck's law.
Microscale Radiative Heat Transfer
Turning now from the field of semiconductor devices, including photovoltaic cells and the like, to the general field of radiative heat transfer, in the classical theory of radiative heat transfer, the radiated power per area and per interval of wavelength of a flat surface in thermal equilibrium with its surrounding is given by Planck's Law. Integration of Planck's Law over all wavelengths yields the Stefan-Boltzman Law for black surfaces. Similarly, this law governs the exchange of energy between two black surfaces.
Planck's law predicts that a large portion of the radiative energy at a given temperature of the radiating body will be around the wavelength of greatest spectral intensity "lambdamax". "Lambdamax" is predicted by the Wien Displacement Law. At shorter wavelengths the power falloff is very rapid whereas at wavelengths greater than lambdamax the falloff is much more gradual. At lower temperatures, lambdamax occurs at longer wavelengths.
In the above classical theory, it is assumed that the distances between radiating surfaces all large compared to the wavelengths of the energy involved. Planck himself imposed this condition on his derivation. Over the last several decades, a small segment of radiative heat transfer theory and experimentation has developed wherein the spaces between radiating solids are on the order of and smaller than the characteristic wavelengths of the radiation exchanged. There is experimental evidence to show that the energy exchange between two surfaces (dielectric to dielectric or metal to metal) separated by a distance of the same order as the wavelength or less can be several times larger than at larger distances, and that the magnitude of this effect increases sharply with decreasing distance. Examples of such experiments are Cravalho, E. G. et. al., November 1967, "Effect of Small Spacings on Radiative Transfer Between Dielectrics", Journal of Heat Transfer, pp.351-358; Hargreaves, C. M., 1973, "Radiative Transfer Between Closely Spaced Bodies", Philips Res. Reports Supplement No.5, pp. 1-80; and Kutateladze, S. S., et. al., August 1978, "Effect of Magnitude of Gap Between Metal Plates on their Thermal Interaction at Cryogenic Temperatures", Sov. Phys. Dokl. 23(8), pp.577-578. Orders of magnitude increase with very small or "microscale" spacings were theoretically predicted by Polder, D. et. al., November 1971, "Theory of Radiative Heat Transfer between Closely Spaced Bodies", Physical Review B, Vol. 4, No. 10, pp.3303-3314 and Levin, M. L. et. al., 1980, "Contribution to the Theory of Heat Exchange Due to a Fluctuating Electromagnetic Field", Sov. Phys. JETP, Vol. 6, pp.1054-1063.
Underlying the present invention, is my novel conceptual insight and discovery that these previously unrelated technologies of thermophotovoltaic energy conversion and of small spacing radiative heat transfer systems could synergistically be combined in such a manner as to enhance the generation of semiconductor carriers (electrons and holes) in semiconductor devices such as photovoltaic cells and the like, receiving radiation, such as photons, from a heated surface, through the use of very small gap juxtaposition of the surfaces of the device and the heated surface.